It is known, e.g. from U.S. Pat. Nos. 3,197,710 and 3,412,460, to form the emitter and conductor regions of such a PNP transistor as concentric zones into which p-type impurities are diffused concurrently with the formation of other components in the same substrate, such as a vertical NPN transistor. The inner and outer diffusion zones representing the emitter and collector regions are concentrically separated by a base region which is part of the epitaxial layer.
In normal operation, the p/n junction existing between the base and collector regions (i.e. along the inner periphery of the generally annular outer diffusion zone) is reverse-biased with resulting formation of a depletion zone in the adjoining base region. When the reverse bias exceeds a certain threshold voltage, the electric field across the oxide layer tends to produce an inversion layer in the underlying epitaxial layer. The likelihood of the formation of such an inversion layer increases with higher open-circuit collector/emitter breakdown voltage V.sub.(BR)CEo. The appearance of this inversion layer depends not only on the applied collector potential and the extent of the depletion zone but also on the charge concentration in the base region which in turn can be modified by the accumulation of charges in and above the overlying portion of the oxide layer. Thus, it has already been proposed to inhibit the development of an inversion layer, which in effect acts as an extension of the emitter region, by enlarging the metallized area constituting the emitter electrode to let that area overlie the base region separating the two diffusion zones. This measure allows the use of increased breakdown voltages V.sub.(BR)CEo without the danger of punch-through with the normal mode of energization of the transistor.
Nevertheless, the risk of punch-through exists in the event of accidental voltage inversion which may happen, for example, when the silicon wafer incorporating the transistor is connected in an external circuit as a replacement of electromechanical components previously utilized therein. In such an event the depletion zone may merge with the inversion layer to cause a destructive short circuit unless the transistor is protected by resistive elements, e.g. diodes, in series with its emitter or collector. The presence of such series elements, however, increases the voltage drop across the semiconductor component during high-current operation to an extent which may be inadmissible in some instances.